1. Field of the Invention
The present invention relates to a medical X-ray image detecting device, more particularly, a compact dental X-ray image detecting device inserted and used in a mouth to photograph teeth.
2. Prior Art
An X-ray photographing system for taking photographs in a mouth, wherein X-rays penetrated a tooth section in a mouth are converted into an optical image on the fluorescent surface in a compact X-ray image detecting device disposed behind the tooth section in the mouth and the optical image is projected to the image pickup surface of a solid-state image pickup device such as a CCD device via a plurality of glass fiber bundles, then the optical image is converted into a digital signal and reproduced on a monitor display, has become to be used for dental treatment.
In a conventional intra-oral (inside-mouth) X-ray radiography system, the position of an X-ray source for irradiating X-rays is set to a target tooth section and a compact X-ray image detecting device is set behind the tooth section. The X-ray tube voltage and current of the X-ray source are then set depending on the conditions of the photographing target section, the size and other factors of a patient by the operator of the photographing system. After these settings, X-rays are irradiated during a specific radiation period predetermined by the X-ray tube voltage and current.
The X-rays irradiated from the X-ray source penetrate the tooth section (target object to be photographed) and are converted into an optical image having a brightness proportionate to the dose of penetrated X-ray radiation to the fluorescent surface on an X-ray fluorescent element. The X-rays which have penetrated the X-ray fluorescent element are not only unnecessary but also harmful to a human body if the dose of exposure increases. In addition, the unnecessary X-rays sometimes cause adverse effects to the CCD device. It is therefore necessary to take countermeasures to prevent the adverse effects of the X-rays.
In addition, it is known that a conventional X-ray image detecting device adopts a method of projecting contracted optical images to the image pickup surface of a CCD device by processing the optical input or output surfaces of a plurality of glass fiber bundles for transmitting optical images so that the input or output surfaces obliquely intersect the cores of the filaments of the glass fiber bundles. As another conventional X-ray image detecting device, the image pickup surface of a CCD device is arranged parallel to the X-ray direction or disposed in a container away from an X-ray penetration area behind the fluorescent surface of a fluorescent element to avoid the adverse effects of X-rays on the CCD device (Laid-open Patent Application No. 2-249537).
As described above, when such conventional methods are used, wherein the CCD device's exposure period for the optical image on the fluorescent surface formed by the X-rays penetrated the tooth section is determined after the X-ray radiation conditions of the X-ray generator are determined according to the experience of an operator, the blackening degree of the X-ray image formed on the monitor display is not constant and the quality of the image changes at each photographing time, occasionally preventing image quality required for diagnosis from being obtained. As a result, rephotographing may frequently be required. This takes longer time for dental diagnosis and increases the dose of X-ray exposure to a patient, causing adverse effects.
The above-mentioned conventional device wherein the glass fiber bundles have surfaces obliquely intersecting the cores of the fiber filaments is conveniently used to contract an image of a target object projected to the fluorescent surface. However, the device lowers the resolution of the X-ray image reproduced, and is thus not necessarily suited for dental treatment.
In addition, until the X-rays reach the fluorescent surface from the X-ray source, a part of the X-rays is scattered when penetrating the inside of a target object such as a tooth tissue, generating scattered X-rays with low energy. Since the CCD device also senses X-rays with low energy of several keV or less and generates an electric signal, the X-ray image picked up by the CCD device and reproduced on the monitor display is subjected to external disturbance from the tooth tissue for example and the clearness of the image is lowered. This kind of scattering of X-rays cannot be avoided even when the CCD device is disposed in the detecting device to try to avoid the X-rays generated straight from the X-ray source as described above. As a result, the scattered X-rays enter the CCD device and disturb the image signal.
Furthermore, in the cases of the conventional X-ray image detecting devices, the X-rays penetrated the CCD device to the backside thereof is irradiated to a human body. This radiation of X-rays must be prevented.